The aim of this research was to study the feasibility of anaerobic digestion of the alga Sargassum muticum with special attention to its biodegradability, potential toxicity caused by its salt content, alga components and intermediate process compounds, and potential limitations to continuous treatment. Specific methane potential (SMP) for three samples of S. muticum collected from the Galician coast (Northwest Spain) at different seasons ranged from 166 to 208 mL CH4/gVS while accumulation of toxic compounds was not observed at alga concentrations of up to 100 gTS/L, except for one of the samples in which inhibition started at 80-100 gTS/L. Continuous digestion is feasible at alga concentration up to 100 gTS/L with methane production rates ranging from 0.14 to 0.26 L CH4/L d at organic loading rates of 3.2 gTS/L d, but SMP dropped to 113-159 mL CH4/gVS. (C) 2015 Elsevier Ltd. All rights reserved.